'^f' Carboxylic acids 



Malic and fumaric acids 



Malic acid and fumaric acid are rapidly labeled during 

 steady-state photosynthesis with €^^^02. These acids are 

 probably formed by reduction of the product of carboxyla- 

 tion of PEPA. In the steady-state experiment that yielded the 

 results shown in Table 1, about 5 per cent of the C^^ uptake 

 rate could be accounted for in the labeling of these two 

 acids. In that experiment very little of the radioactivity finds 

 its way into succinic acid. It would thus appear that, if malic 

 and fumaric acids are labeled by reductive carboxylation of 

 PEPA, either (1) the reaction sequence is highly reversible, 

 leading to exchange labeling, or (2) the malic and succinic 

 acids are converted to other compounds by as-yet-undeter- 

 mined paths. 



The probability of labeling via exchange (1) may be 

 answered by a thermodynamic argument. Under the condi- 

 tions existing in the chloroplast during photosynthesis, the 

 actual free energy change accompanying the conversion of 

 PEPA, CO2, TPNH, and either ADP or IDP to malic acid, 



37 



